RAPPORTS. XIII C2: THOMPSON 



— 6 — 



cm 

 120 



100 



If we now consider the four curves (figures 1—4) we see that in each of them the 

 several annual groups are marked with great clearness, and we can, at least for the first three 

 or four annual groups, determine within a centimetre or two the modal size, that is to say the 

 cusp of the curve, or the size which corresponds to the greatest number of individual fish. 



In figure 5 1 have attempted 

 to lay down these points, as 

 obtained from the former se- 

 ries of curves, in a new dia- 

 gram where size in centime- 

 tres is plotted against time, in 

 other words where the diagram 

 represents the rate of growth 

 of the fish. It will be seen 

 that the points follow one an- 

 other in a very orderly way, 

 and show us an undoubtedly 

 close approximation to the rate 

 of growth of the Cod during its 

 first four years. The net result 

 expressed in tabular form is 

 as follows: 



Approximate Mean 

 Sizes of Cod in centi- 

 curve based on observations 



80 



60 



40 



20 



Fig. 5. Curve of the rate of growth of Cod, 

 in the Firth of Forth and Moray Firth. 



metres from — VI Years of Age; from a smoothed 

 in the Firth of Forth and Moray Firth. 



Annual Group 



February . . — 



May — 



August .... — 



November. . 1 1 



I 



15 

 20 



25 

 29 



II 



33 

 38 

 43 



47 



in 



52 

 57 

 61 

 66 



IV 



71 

 (75) 

 (79) 

 (83) 



V 



(85) 

 (88) 



(91) 

 (94) 



VI 



96 

 97 



Looking back upon the same curves, we see that at all seasons Cod of more than 

 about 70 or 80 centimetres, or say about 30 inches long, are exceedingly scarce in the 

 Firth of Forth, and the number goes on diminishing with increasing size. The groups 

 that are present in great abundance in the trawl net are the small and large "Codling", 

 of from 30 to 50 or 60 centimetres (12 to 24 inches), and these our curves show to be 

 fish of about two and three years old. 



Let us consider the four seasonal curves a little more closely. In the January to 

 March curve (which for brevity's sake we take to be typical of the month of February), 

 we see a well-marked group of fish captured in the small mesh net, which vary in size 

 from about 8 to 26 centimetres, with a sharp cusp at 15 centimetres, or about 6 inches 

 (fig. 1). These we cannot look upon otherwise than as fish of the foregoing spawning 

 season; in other words, they are just about one year old. We shall denominate these 

 group I, and shall in like manner date our other annual groups from the beginning of 

 the calendar year, in spite of the fact that this method is by no means that generally 



